Add ERROR_IF checks to CONCAT operator

 * Add ERROR_IF checks to ensure that inputs have the correct rank,
dimension lengths, and a suitable axis value
 * Make REVERSE REQUIRES check into an ERROR_IF
 * Some minor typo/formatting fixes

Change-Id: Ie133788ca7e1deab194ba5ef97e47c39cdd170b6
Signed-off-by: Matthew Haddon <matthew.haddon@arm.com>